It is often necessary to measure the density and/or moisture profile in the longitudinal direction of a sample. Important examples of this are the density measurement in a cigarette production line, the density measurement or moisture measurement in a woolen fibre, a plastic profile or other elongated materials. However, such measurements also have to be carried out with relatively short samples. If one wishes to determine the density and/or moisture profile in a wood-fibre board or chipboard, then a cylindrical core is often drilled out, along whose cylinder axis the density or moisture profile is then recorded.
Various methods are known for such measurements of density and moisture profiles.
Cigarettes are produced in large quantities by cigarette machines (up to 15,000 cigarettes per minute). It is necessary to measure the density exactly in order, in the process, to obtain optimum tobacco compression at one or both cigarette ends. Nowadays, this is carried out as a rule using gamma rays. The damping of the highly energetic photons is in this case dependent not only on the density but also on the composition of the material to be measured, and, in particular, on the water content of the tobacco as well. Density measurement is thus subject to uncertainties. Furthermore, efforts are, of course, being made to avoid the dangerous gamma radiation measurement technique. The use of infrared radiation for such measurements has the disadvantage of severe sensitivity to surface effects. It is thus not possible to achieve absolute density values, but only percentage statements relative to a maximum value for one tobacco type.
Density profile measurement in the case of wood-fibre boards is also an important process parameter in terms of quality assurance. The most important measurement method to date is also the gamma radiation method developed in the mid-1970s. Once again, it is disadvantageous here that the sensitivity to product moisture also limits the measurement accuracy of the density measurement.
It is known for both the density and the moisture of materials to be measured using microwaves, the products to be investigated being inserted into a microwave resonator (EP 0 468 023 B1). However, the disadvantage of this already known method is that the resonators and samples must be relatively large, so that it is not possible to measure a density or moisture profile with millimetric resolution. The reason for this is that the microwave frequency cannot be increased indefinitely since, otherwise, it would no longer be possible to obtain accurate readings. The measurements should thus be carried out with microwaves in a frequency band from 0.5 GHz up to a maximum of 15 GHz, which corresponds to wavelengths of 60 cm to 2 cm. One particularly useful frequency in this case is 2.5 GHz, which corresponds to a wavelength of 12 cm. In this case, the dimensions of the microwave resonators are normally in the same order of magnitude as one wavelength.
A microwave resonator of the type mentioned initially is known which has internally a projection through which the material to be measured moves (EP 0 292 571 claim 1). This microwave resonator also makes it possible to carry out measurements at a relatively low frequency. The actual measurement range is in this case relatively small owing to the short distance between the projection end and the opposite cavity wall. However, the field is highly inhomogeneous. The microwave field is very strong in the center and decreases considerably toward the edge so that, on the one hand, it is impossible to carry out a uniform measurement over the entire sample and, on the other hand, fluctuations occur in the readings if the sample is also moving in the transverse direction.